1. Field of the Invention
The present invention concerns particles of boehmite and particularly to boehmite particles that are particularly adapted to use as fillers for thermoplastic resins.
2. Description of the Related Art
Boehmite has long been recognized as a suitable filler for thermoplastic resins in view of the fine particle size and the relative inertness to the polymer in which it is dispersed. It produces good improvements in modulus and thermal behavior and has even been proposed as a flame retardant in U.S. Pat. No. 6,143,816.
The preferred form as recited in a number of US Patents such as U.S. Pat. Nos. 5,360,680 and 5,401,703 is that of fine flakes which were felt to be preferable since earlier fine grained boehmite powders produced by a milling process tended to be globular which is less effective as a filler material. Other methods of making fine boehmite particles caused other problems. For example pyrolysis tends to produce a range of contaminants of the boehmite product and conventional hydrothermal treatments tend to produce tight agglomerates of rhombic prisms which are not easily separated. Refined hydrothermal treatments of aluminum hydroxides such as that of U.S. Pat. No. 5,194,243, (which built on earlier work described in U.S. Pat. Nos. 2,763,620; 2,915,475; 3,385,663; 4,117,105; and 4,344,928), introduced the concept of seeding the conversion of the boehmite pre-cursor using boehmite seeds. This was found to require a high level of seeding to be effective using at least 15% of boehmite seed in order to produce a submicron boehmite dispersion with essentially equiaxed particles.
All such efforts produced flaky or equiaxed particles of boehmite which did not readily disperse as individual particles in a polymer unless previously treated with a surface active agent designed to enhance such dispersibility. Typical surface active agents used include amino-silanes which have the capability of bonding to the hydrophilic surfaces of the particles giving them a surface more compatible with the organic polymer in which they were to be dispersed. This avoided the clumping that tended to occur without such a treatment. This tendency becomes greater with decreasing size of the particles because of the increasing surface energy that naturally occurs with diminishing size.
The present invention provides a novel form of boehmite that can be readily dispersed in a thermoplastic polymer at the nano-particle level without the need for any surface treatment and which provide a significantly improved blend of properties by comparison with the prior art boehmite filler materials.